1. Field of the Invention
The present invention relates to a heat treatment device used for PEB (post exposure baking), which bakes a process material film applied on a substrate such as a semiconductor wafer or glass substrate for liquid crystal display after exposure but before development, and such a heat treatment method.
2. Description of the Related Art
In the manufacture of semiconductor devices, a desired circuit pattern is formed on each semiconductor wafer using the so-called photolithography process. In the photolithography, each semiconductor wafer is subjected to a series of processes including washing, dry baking, adhesion (process to make the wafer water-repellent), resist application, pre-baking, exposure, post-exposure baking (PEB), development and post-baking, which are carried out in this order.
The above-described photolithography process has to deal with a very thin line width of the circuit pattern. Therefore, a chemically amplified resist that contains a polymer that transforms to be alkali-soluble in the presence of acid and an acid generator is used.
By referring to FIGS. 1A and 1B, a conventional pattern formation method that uses a chemical amplified resist will now be described. First, a chemically amplified resist 18 is applied to a wafer W. The resist 18 is, for example, a KrF resist (a type of chemically amplified resist) that contains an onium salt as the acid generator, and polyvinylphenol protected by a tert-butoxycarbonyl group (t-BOC), as the polymer.
Next, as shown in FIG. 1A, a KrF excimer laser beam 15 is applied to the resist 18 via a mask 16, and thus the resist 18 is exposed to form a pattern. When the KrF excimer laser beam 15 is applied to the resist 18, an acid (H+) or positive ion is generated from the acid generator in the resist 18.
After the exposure but before the development, when the resist 18 is heated in the PEB step, the acid (H+) reacts with the t-BOC group. Due to this reaction, the t-BOC group is decomposed to generate a phenolic hydroxide group and an acid (H+). Due to the decomposition of the t-BOC group, the acid (H+) is newly generated and thus the number of positive ions (H+) increases. Consequently, the reaction proceeds in a chain reaction manner (chemical amplification reaction). As described, in the chemically amplified resist 18, the positive ions (H+) serve as a catalyst, and thereby the sensitivity is improved.
In the developing step, which follows, an alkali developing solution (for example, TMAH solution) is brought into contact with the resist 18 to dissolve an exposed section 18a, which is now rendered alkali-soluble, and remove it. Thus, a positive-type resist pattern 19 as shown in FIG. 1B is obtained.
It should be noted here that after carrying out the post-exposure baking (PEB), the resist must be cooled down in a short period of time. Otherwise, the chemical amplification reaction proceeds more than needed, causing a problem of creating an adverse affect on the width of the pattern line ultimately formed. Further, if the time period until the cooling process starts after the PEB varies from one occasion to another, the width of the line varies from one wafer W to another accordingly, which is not desired.
Jpn. Pat. Appln. KOKAI Publication No. 2001-85323 discloses such a technique that substrates to be processed are cooled down while they are carried by means of a cooling unit equipped in a carriage unit for carrying the substrates. With this conventional technique, the cooling process can be started within such a short time after the heating process of the wafers, and therefore the temperature distribution within the wafer surface can be made even, thereby making it possible to improve the yield of the products.
Incidentally, the post-exposure pre-developing baking (PEB) is a process that is carried out to promote the reaction of the acid in the chemically amplified resist, and therefore the decomposition of the protection group (t-BOC group) by the acid (H+) is promoted. For example, as shown in FIG. 2A, in the initial stage of the reaction, the acid (H+) diffuses in random directions within the range of a mask pattern width d1, to attack the protection group (t-BOC group).
However, the diffusion of the acid (H+) proceeds not only in the film thickness direction (Z-direction) but also in the film surface direction (X or Y direction) or a diagonal direction. Consequently, the diffusion of the acid (H+) proceeds beyond the mask pattern width d1 (the target pattern line width) during the PEB process as shown in FIG. 2B. FIG. 2C shows the results, in which a pattern line width d2 after the development becomes larger than the mask pattern width d1.
As described above, the conventional technique cannot control the progression of the chemical amplification reaction during the post-exposure and pre-development baking (PEB), and therefore it entails such a drawback that the line width of a circuit pattern ultimately obtained is larger than the target pattern line width.